Formulations suitable to provide polyurethane (PU) and polyurethanes obtained by reacting said formulations are well known in the art. Polyurethane, mainly flexible and rigid foams, is used in transportation, refrigeration, home furnishing, building and construction, marine, and business machines. For many of these products, it is necessary to add flame retardants to the polyurethane. However, since most of the end applications are internal, polyurethane is in a critical situation and directly subjected to increasingly stringent regulations which on one side require high fire safety standards and on the other side limit the use of potentially toxic but extremely effective flame retardants.
This is the case of halogenated flame retardants. Halogenated fire retardants are generally very effective, requiring relatively small quantities to be added in the final product in order to obtain outstanding flame retardant properties, but they have been included in the list of priority pollutant as a hazardous priority pollutant, and their use is being limited.
Another point to be considered is that flame retardants can actually reduce the product's physical properties, cause processing problems and shorten the useful life of a product if they are not compatible with the material itself or other additives. Some halogenated flame retardants are very effective at concentrations of a few percent whereas many inorganic flame retardants require concentrations of 30% or higher, thus degrading the mechanical value of the plastic part. When more environmental-friendly flame retardants are used in the place of halogenated compounds, such as inorganics or melamine, a compromise has to be found between the achieving of acceptable fire properties and the high load required, which is detrimental to the material performance.
This is especially important for PU foams that, on one hand are more flammable because of their cellular structure and on the other hand are strongly affected by the addition of flame retardant which can affect the complex cell structure of the foam, thus reducing the final mechanical and insulation properties.
EP 0512629 discloses the use of zinc borate in combination with encapsulated ammonium polyphosphate in thermoplastic urethanes. The flame retardant combination must contain, in addition to zinc borate, a “carbonific” (polyhydric char-forming) compound such as pentaerythritol. However, there remains a need for halogen-free fire retardant formulations able to produce improved fire performances at reduced loading of additives.
It is an object of the present invention to improve the fire resistance of polyurethane products.